Some animals, like certain insects, spiders, and plants, die immediately after mating. But what about mammals? A new paper shows that the males of certain mouse-like marsupials face imminent death after having sex — an evolved trait that arose on account of extreme promiscuity.

Photo by Mel Williams.

Biologists call it suicidal reproduction, or semelparity (meaning “once to beget”) — and it happens when one or both sexes of a species die after doing the nasty. It sounds crazy, and even maladaptive from an evolutionary perspective, but it’s a strategy that points to the extreme lengths the selfish gene will go to ensure replication. Suicidal reproduction appears in certain species as a way to put all available resources into maximizing reproduction — albeit at the expense of future life.

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A prime example of semelparity in nature is Pacific salmon, who swim to the freshwater streams of their birth, spawn, and promptly die. Female octopi also die soon after giving birth on account of exhaustion, leaving their offspring to fend for themselves. Other examples include butterflies, cicadas, mayflies, and many arachnids, like certain black widow species (e.g., Australia’s redback), where the female cannibalizes the male after mating.

Fatal Attraction

As for mammals, it’s only been observed in four different genera, all of them closely related. According to the new study, which now appears in PNAS, suicidal sex occurs in certain insect-eating mouse-like marsupials, namely those belonging to the generas Antechinus, Phascogale, Dasykaluta, and Parantechinus.

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Unfortunately for the males of these species, their stress hormones shoot through the roof during the breeding season, causing immune system collapse, catastrophic hemorrhaging, infections, and ultimately, death after mating. And this holds true for all males.

So why does this happen? It’s a trait that evolved owing to a number of conspiring factors, including limited availability of food, short mating seasons, extreme promiscuity, and most importantly, postcopulatory selection (i.e. sperm competition). Basically, the males of these species pour everything they’ve got into sperm production — sperm that then battles it out inside the female’s reproductive tract.

”Horrible to See”

According to the researchers, a team lead by Diana Fisher, it’s a reproductive strategy that can be traced back to the environment and ridiculously short breeding seasons. These insectivores live in regions like Australia and Papua New Guinea — areas in which the predictability of insect abundance has resulted in short, synchronized single mating seasons; females are at their reproductive best when their reproductive cycles coincide with maximum food abundance.

But during these short and intense breeding seasons, the marsupials become extremely horny and promiscuous. In the 12 species of Antechinus, pairs have been observed to mate for up to 14 hours at a time. And they exchange partners with great regularity.

This has led to sperm competition and a rather bizarre adaptation (if you can call it that). Just prior to mating, and when they’re just 11 months old, the males completely stop producing sperm and their testes disintegrate. But because they still need to mate, they store the last bit of sperm in a narrow coiled tube called an epididymis.

But now the clock is ticking; the sperm have only a short time to live. So, for the next week or two, the males and females mate like crazy. The males, who then start to suffer from lethal immune system collapse, die before their offspring are even born.

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And according to the scientists, it ain’t pretty. The males lose their fur and can develop ulcerations and gangrene. It’s a mass synchronized suicide that the researchers describe as something that’s “horrible to see.”

But as far as evolution is concerned, it’s just an evolved form of self-sacrifice.